[PATCH] D62825: [C++2a] Add __builtin_bit_cast, used to implement std::bit_cast

[John McCall via Phabricator via llvm-commits]

rjmccall added a comment.

In D62825#1542374 <https://reviews.llvm.org/D62825#1542374>, @rsmith wrote:

> In D62825#1542309 <https://reviews.llvm.org/D62825#1542309>, @rjmccall wrote:
>
> > In D62825#1542301 <https://reviews.llvm.org/D62825#1542301>, @rsmith wrote:
> >
> > > In D62825#1542247 <https://reviews.llvm.org/D62825#1542247>, @rjmccall wrote:
> > >
> > > > In what sense is the bit-pattern of a null pointer indeterminate?
> > >
> > >
> > > The problem is not null pointers, it's `nullptr_t`, which is required to have the same size and alignment as `void*` but which comprises only padding bits. (Loads of `nullptr_t` are not even permitted to touch memory...).
> >
> >
> > I mean, I know this is C++ and the committee loves tying itself in knots to make the language unnecessarily unusable, but surely the semantics of bitcasting an r-value of type `nullptr_t` are intended to be equivalent to bitcasting an r-value of type `void*` that happens to be a null pointer.
>
>
> I don't follow -- why would they be? `bit_cast` reads the object representation, which for `nullptr_t` is likely to be uninitialized, because the type contains only padding bits. (Note that there is formally no such thing as "bitcasting an rvalue". `bit_cast` takes an lvalue, and reinterprets its storage.)


I agree that the problem is that the object representation of `nullptr_t` is wrong, but it seems absurd to me that we're going to bake in an absurd special case (from the user's perspective) to `bit_cast` because we consider that representation unfixable.


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